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United States Patent O 3,521,126 ROLLER CHARGING APPARATUS Daniel B. Granzow, Arlington Heights, and Karl K. Klessig, Hoffman Estates, Ill., and Richard J. Pleitt, Mililintown, Pa., assignors to Addressograph-Multigraph Corporation, Mount Prospect, Ind., a corporation of Delaware Filed July 28, 1967, Ser. No. 656,820 Int. Cl. H01b 1/00 U.S. Cl. 317-3 2 Claims ABSTRACT OF THE DISCLOSURE Driven roller electrodes connected to a DC power supply simultaneously move and charge a photoelectrostatic member. One roller is formed of a ceramic material having metal particles dispersed therein. This material comprises a major portion of aluminum oxide or baryllium oxideand a minor portion of a reducible metal ion containing additive, such as iron oxide, tin oxide, copper oxide, zinc oxide, manganese oxide or silicon carbide. Reduction of the additive provides the metal particles which are present in an amount sucient to render the material semi-conductive so that its resistivity is in the range of from about lOl-109 ohm-centimeters as measured over an applied voltage within the range of from 2-8 kilovolts.

BACKGROUND OF THE INVENTION This invention relates to improvements in conventional roller 'charging apparatus, and more particularly concerns a roller charging apparatus employing a novel semi-conductive ceramic roller electrode.

As is disclosed in U.S. Pat. No. 2,980,834, roller charging devices are well known means for uniformly charging a photoelectrostatic recording member. These conventional roller charging devicesy include a pair of driven roller electrodes connected to a DC power supply. One of the rollers of the apparatus is formed of a semi-conductive material, such as rubber or a ceramic, mounted on a conductive shaft. The other is made of a highly conductive material such as, for example, metal.

Known rubber and ceramic rollers are seriously deficient insofar as their electrical properties are concerned. Namely, the rubber roller, although initially functioning in a satisfactory manner, breaks down and becomes either conductive or highly resistive within a few hours operating time under the influence of the ionized and ozone laden atmosphere in the vicinity of the charging apparatus. If the resistivity of the rubber decreases below about 106 ohm-centimeters, it is considered too conductive. Whereas, if its resistivity exceeds 1010 ohm-centimeters, it is considered too resistive. As a result, the apparatus is no longer able to charge the photoelectrostatic recording member in a satisfactory manner. Similarly, known ceramic rollers, although they do not usually undergo degradation in electrical properties to the degree exhibited by rubber rollers, are also generally too conductive or too resistive. Charging apparatus using such conventional ceramic rollers are incapable of laying down on the surface of the recording member a charge of a high enough voltage level. As a result, the photoelectrostatic recording member may not be imagable or the density of the image is very poor. Due to these disadvantages, known roller charging devices have not been used in commercial photoelectrostatic copying equipment.

SUMMARY OF THE INVENTION Accordingly, it is one object of the present invention to provide an improved roller charging apparatus employing a roller made of a semi-conductive material that does not break down and become conductive or highly resistive in the presence of an ionized and ozone laden atmosphere.

It is another object of this invention to provide an improved roller charging apparatus equipped with a novel, semi-conductive ceramic roller that is capable of consistently producing on a photoelectrostatic recording member a high-level, uniform electrostatic charge.

The improved roller charging apparatus of this invention is equipped with a pair of driven roller electrodes for charging the member and moving it along a predetermined path, and power supply means connected to the roller electrodes for establishing an electric iield therebetween which ionizes the atmosphere between and about the nip of the roller electrodes. The improvement is that one of the roller electrodes is formed of a ceramic material having a resistivity within the' range of from about 107 to 109 ohm-centimeters as measured over an applied voltage with the range of from 2-8 kilovolts. We have discovered that, in order to have ceramic rollers function properly, the resistivity of the ceramic material employed must be within the above mentioned resistivity range. This limitation is critical.

The best way of providing a ceramic material which has the proper resistivity is to add, during preparation of the ceramic, highly conductive substances which regulate the overall resistivity of the ceramic material. A preferred form of ceramic material, commonly referred to in ceramic art as a cermet, has randomly shaped sub-micron size metallic particles distributed throughout in an amount suilicient to render said ceramic semi-conductive so that its resistivity is within the aforesaid critical range. One type of cermet which can be used comprises a major amount of aluminum oxide or beryllium oxide and a minor amount of metal ion containing additive that can readily be reduced to provide the metallic particles. Additives, such as iron oxide, tin oxide, copper oxide, zinc oxide, manganese oxide and silicon carbide have been found to be most suitable. These additives are preferably present in an amount of from about 10%-25% by weight of the material.

In accordance with the preferred method of preparing the ceramic, a readily reducible additive or mixture of additives, such as already mentioned, are admixed with the major constituent and reduced until a sufficient amount of elemental metallic material is available so that the resistivity of the ceramic is within the prescribed range.

Although free metallic particles may be added to the ceramic during preparation, this method is usually not preferred because it is extremely difficult to control the resistivity with any degree of consistencey. However, the addition of a trace amount of a free metal, such as, for example, manganese, may be beneficial.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawings wherein:

FIG. 1 is an elevational view partially in section of the preferred embodiment of this invention; and

FIG. 2 is a sectional view taken along line 2-2 of FIG. l.

DESCRIPTION OF PREFERRED EMBODIMENT The improved roller charging apparatus of this invention, as shown in FIGS. 1 and 2, is provided with a roller electrode 12 having a novel ceramic shell 14. In accordance with the critical feature of this invention the ceramic shell material has a resistivity within the range of from about 10'-109 ohm-centimeters as measured over an applied voltage ranging from 2-8 kilovolts. The preferred resistivity is about 108 ohm-centimeters as measured at about 5 kilovolts. It will be appreciated that the measured resistivity of the ceramic material, in addition to being a property of the material per se, is also dependent on the applied voltage at which the measurement is taken. Accordingly, the resistivity is measured over an applied voltage ranging from 2-8 kilovolts.

lIn measuring the resistivity of the material in accordance with standard techniques, a sample of known crosssectional area and length is connected lengthwise between the positive and negative terminals of a power supply having a known output voltage in the range of from 2-8 kilovolts. A current meter is also connected across the sample so that current flow through the length of the sample is measured. The resistivity is then calculated using the following formula:

wherein p=resistivity in ohm-centimeters,

A=the cross-sectional area of the sample in square centimeters,

L=the length of the sample in centimeters,

I=the measured current in amps, and

V=the applied voltage.

Resstivity measurements are taken at about 72 F.

A wide variety of materials can be used in the preparation of the ceramic shell 14, but care must be taken to insure the resistivity of the ceramic material is within the critical range.

In preparing a suitable roller shell 14, such as shown in FIGS. 1 and 2, a major portion of a ceramic such as, for example, aluminum oxide and a minor portion of an additive such as, for example, iron oxide, are blended together with a small amount of water to form a paste. The paste is sprayed dry to 4form a fine powder which is then placed in a mold and packed down by applying from both ends of the mold a pressure of about 10,000 pounds per square inch. The shell is then removed from the mold and baked for several hours (approximately 8 hours) in a reducing environment such as an atmosphere containing hydrogen. During the baking or firing process the temperature is gradually increased to about 1000 C., maintained at this level for a given period and then gradually reduced. This control baking process regulates the degree to which the additive is reduced. The tiring period, temperature, and rate of heating and cooling are varied depending on the material used in order to produce a ceramic having the proper resistivity. The iron oxide is, as a result of the firing, partially reduced to provide submicron size metal particles, which are schematically i1- lustrated in FIG. 1v as 13. Although it is believed that the aluminum oxide is not reduced, it is possible that this material is partially reduced during the firing process to provide trace amounts of free aluminum metal.

Other suitable additives such as, for example, copper oxide, tin oxide., zinc oxide, manganese oxide and silicon carbide, or mixtures thereof, can be used.

It was noted that not all the ceramic rollers of this invention perform equally well under high humidity conditions. However, aluminum oxide rollers doped with iron oxide give reliable performance, regardless of humidity conditions. For this reason rollers of this type are preferred.

It was further noted that charging apparatus employing a ceramic roller having a resistivity in the range of from about 1 l08 to 3 l08 ohm-centimeters as measured at about 5 kilovolts can feed a photoelectrostatic member at varying rates anywhere from about 0-40 feet per minute and still lay down a charge having the same voltage level (approximately 600 volts), regardless of the speed the member is being moved. Accordingly, ceramic rollers of this resistivity are preferred.

Referring to FIGS. 1 and 2, there is shown the roller charging apparatus 10 including roller 12 and a metal idler roller 16. Rollers 12 and 16 are mounted in driving engagement between sidewalls 18 and 20, with roller 16 being held by brackets 22 and 24. This roller 16 can have the same or a different size diameter than roller 12, and more than one such metal idler roller can be in driving engagement with roller 12.

Roller 12 is mounted on a conductive drive shaft 34 which makes electrical contact along substantially the entire length of the roller, thereby insuring the establishment of an effective corona charging zone between and about the nip of rollers 12 and 16. Shaft 34 is received in non-conductive bearings 30 and 32 which are supported in the sidewalls 18 and 20. A pulley 40, mounted by means of an insulating bushing 42 and driven by motor 44, is connected to one end of shaft 34. When the motor 44 is turned on, the rollers 12 and 16 are driven. Other drive arrangements, too numerous to describe in detail, can be employed which are equivalent to the structure set forth and are contemplated to be within the scope of this invention.

A brush member 46 engaging the other end of shaft 34 is connected to the negative terminal of a 5kilovolt DC power supply 48. Preferably, the metal idler roller 16 is connected to ground so that the potential between the rollers 12 and 16 is about 5 kilovolts.

'Ihe photoelectrostatic member to be charged is fed into the nip of the rollers 12 and 16, whereupon the member is simultaneously advanced and charged. Although an ionized ozone atmosphere is created at the charging zone between and about the nip of the rollers 12 and 16, it does not substantially alter the electrical properties of the ceramic material of this invention. As a result, the roller 12 can be used for an indefinite period and need hardly ever be replaced. Since ceramic roller 12 has the proper resistivity, the photoelectrostatic recording member is charged to the proper level.

It is apparent that certain modifications can be made in the roller charging apparatus of this invention without departing from the spirit and scope thereof. Accordingly, the above description and accompanying drawings should be interpreted in an illustrative rather than limiting sense.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. An improved charging apparatus of the type that applies a uniform electrostatic charge to the surface of a photoelectrostatic recording member, said apparatus having a pair of driven roller electrodes for charging the member and moving it along a predetermined path, and power supply means connected to said roller electrodes for ionizing the atmosphere between and about the nip of the rollers, the improvement wherein one of the roller electrodes comprises a ceramic material having a resistance within the range of from about 10FI to 109 ohm-centimeters as measured over an applied voltage within the range of 2-8 kilovolts, wherein said ceramic material contains sub-micron size elemental metallic particles which are uniformly distributed throughout, and wherein a major amount of said ceramic material is selected from the group consisting of aluminum oxide and beryllium oxide, and a minor amount of an additive containing a reducible metal cation selected from the group consisting of iron, tin, copper, zinc, manganese and silicon, said additive being partially reduced to render said metal cation reduced to its elemental form.

2. The apparatus as defined in claim 1, wherein said 4/1961 Tregay 317-262 3/1961 Harris 252-519 X J D MILLER, Primary Examiner W. I. SMITH, Assistant Examiner U.S. C1. X.R. 317-262 

